(1) Field of the Invention
The present invention relates generally to liquid monitoring systems and, more particularly, to a system for precisely monitoring liquid in a storage tank for the detection of leaks and inventory control.
(2) Description of the Prior Art
Underground fuel storage tanks are used extensively in the service station industry. Leaks from these tanks can result in the escape of thousands of gallons of gasoline or fuel oil which may contaminate the ground water. It is estimated that there are about one million of these underground gasoline storage tanks in the United States. Approximately one-third of these tanks are 20 years old or older. Accordingly, the risk of leaks from a large number of these tanks due to corrosion or structural failure continues to increase. In addition, since the tanks are buried underneath the surface of the earth, such leaks are not easily detectable by visual inspections and inventory control data is usually inadequate for determining existence of any except large leaks.
One method now used to detect tank leaks is to plug all but one opening in tank being tested; connect a "standpipe" to the one unplugged opening; and fill the tank more than the normal maximum amount until the level of liquid in the tank rises into the "standpipe" to a height of several feet above ground level. If the liquid begins to drop in the "standpipe", it is assumed that there must be a leak in the system.
A second, similar test method is to fill the tank with liquid; plug all but one opening of the tank; and pressurize the interior of the tank with air through the one unplugged opening. The unplugged opening then is plugged and the air pressure within the tank is monitored over time. If the air pressure within the tank drops, there may be a leak in the system.
It is readily apparent that these measurement methods are fraught with problems. For example, as will be more clearly explained later, measurement of changes in tank volume are most dependent on temperature when the tank is full or nearly so. Also, tank distortion or air bubbles will cause significant errors. Furthermore, such methods are expensive since the station must be shut down during the test for considerable periods, require a trained operator, and do not continuously monitor the tank in order to provide early detection of leaks.
The current industry standard for threshold detection of leakage has been established by the Environmental Protection Agency (EPA) as 0.05 gallons per hour, regardless of tank size. Many variables, such as changes in the volume of air and liquid due to changes in temperature, make it difficult to determine conclusively the existence of such a small leak.
The temperature of the fluid in an underground tank, for example, can change continually at rates of up to 0.01 degrees F per hour. Changes of this magnitude will cause a volumetric change of up to 0.06 gallons per hour in 8,000 gallons of gasoline. Consequently, a slight change in the temperature of the contents of a tank will produce a volume change which is greater than the amount of leakage which is sought to be detected. Therefore, prior art leak detecting systems usually employ elaborate temperature measuring systems, in addition to the tank level measuring system, to allow compensation for temperature changes.
In order to eliminate or minimize the effect of the variation in volumetric change of the tank contents due to temperature changes, it has been known to measure the mass of the liquid in the storage tank by use of Archimedes' buoyancy principle. Examples of such devices are set forth in U.S. Pat. No. 4,387,778 and 4,244,218 issued to Wohrl and 4,281,534 and 4,300,388 (now Reissue 31,884) issued to Hansel et al.
Wohrl '218 noted that there was a relationship between the cross-sectional area of the displacer and the tank at any fluid level. Furthermore, Wohrl recognized that there was an advantage to specially shaping the displacer to "match" a particular storage tank in order to provide a measurement output which is a linear function of mass for that tank. Specifically, Wohrl '218 considered that the cross-sectional area of the displacer at any fluid level within the operational range of the fluid measuring device should have a constant ratio to the cross-sectional area of the tank at the same fluid level. Thus, Wohrl '218 teaches that it is only necessary to shape the displacer over the operational range of the measuring device.
An upstanding cylindrical tank is an ideal application for measuring the mass of the liquid in the storage tank by use of Archimedes' buoyancy principle since it is simple to fabricate a cylindrical displacer. However, most underground storage tanks are not upright cylinders but are laid horizontally and may have domed ends. U.S. Pat. No. 4,646,560, issued to Maresca et al., provides a particularly detailed description of the variation of total volume of a conventional underground storage tank as a function of depth for tanks of between 4,000 and 10,000 gallons. Maresca does not apply this teaching to a "force on displacer" measurement system, however, Maresca appears to appreciate that special shaping must be over the entire tank for the measurement device to be insensitive to temperature.
However, because of uncertainties in the displacer and tank geometry, the principle method of calibration, as taught by Wohrl '218 and others, is to either first fill or empty the tank and then either progressively remove or add fluid in discrete steps with a known amount of fluid while comparing this with the change in fluid level indicated by the tank monitor. However, such a procedure generally involves making a large number of calibration measurements. In addition, the tank may require recalibration if a sensor is replaced.
Thus, there remains a need for a new and improved tank monitor which is sufficiently sensitive, regardless of temperature changes, so as to detect leaks of 0.05 gallons per hour or less in underground storage tanks having a volume of from 500 hundred to 20 thousand gallons or more.